The present invention relates to a process for treating papermaking fabrics, with the primary concern being the treatment of dryer fabrics or felts.
A papermaking machine has three basic sections: fourdrinier or paperforming section, presses and dryers. In each of these sections different types of papermaking fabrics are utilized. The fourdrinier section uses fourdrinier fabrics or paperforming fabrics, the presses use wet felts and the dryers use dryer fabrics or felts. Although the treatment process of the present invention can be used for treating fabrics employed in any of these sections, the process has been primarily developed for treating the dryer fabrics.
During the operation of a papermaking machine, after the paper web has been formed by the fourdrinier section and some of the moisture removed by the press section, the wet paper web is conveyed around the circumference of a plurality of drying cylinders. The wet web, however, is often too weak to support itself, especially during the early stages of drying. Thus, dryer fabrics are employed for conveying the paper web through the drying section. Two exemplary embodiments of such dryer sections are illustrated in FIGS. 1A and 1B of the drawings. In those Figures, drums 1a through 1g are the drying cylinders, 2a, 2b and 2c are the dryer fabrics and 3 is the wet paper web that is being transported through the drying section. Another function of the dryer fabric is to press the sheet tightly against the cylinder surface thereby increasing the heat transfer between the cylinder and the paper. As the wet paper web moves through the drying section, the heat of the drums causes the moisture within the paper to evaporate. The water vapor evaporates through the openings in the woven dryer fabric.
In recent years, the speeds at which the dryer cylinders are operated have been significantly increased. Such increases in speed have led to certain problems in the operation of such dryer sections, especially with that type of configuration shown in FIG. 1. With increased speeds, air currents are created between the cylinders which, due to high air permeability of the dryer fabric, causes the fabric and the paper web to flutter. Such fluttering can cause stretching of the edges of the paper web, especially with thin papers, which destroys the quality of the paper being produced. In extreme cases, the fluttering also can lead to breaking the paper web thereby necessitating shut-down of the production operation.
The dryer fabrics that are commonly used in the papermaking machine are generally woven with multi-filament and mono-filament yarns. Occasionally, glass yarns are also employed. The resulting woven fabric lacks sufficient rigidity. In order to increase the rigidity, such fabrics have been coated with a liquid resin mixture by a kiss coating process. The resin coating also improves the wearing characteristics of the fabrics.
A kiss coating operation is illustrated in FIG. 2. In accordance with this process, a liquid, i.e., low viscosity resin mixture 7 is applied to a dryer fabric 4. Kiss roller 5, which is rotated in a direction opposite the direction of movement of dryer fabric 4, is coated with the liquid resin mixture as it passes through trough 6. The amount of liquid that is applied can be varied by changing the speed of the kiss roller as well as changing the relative speed between the fabric and the kiss roller. The viscosity of the liquid resin mixture that is employed in this process is on the order of between 80 and 200 centipoise.
Typically, before being treated the woven dryer fabric has an air permeability level of between 175 and 800 CFM. In order to significantly reduce the air permeability of a particular dryer fabric, e.g., to a range of approximately 75 CFM, when utilizing the kiss coating process, it was necessary to apply a plurality of coatings to the fabric. Often, it could take up to 25 coating applications before the air permeability of the dryer fabric was reduced to the desired level. In carrying out the plurality of coating operations, after completing each resin application, it was necessary to dry the fabric and then measure the air permeability value to determine if that value had been reduced to a satisfactory level. That process then had to be repeated again until the desired air permeability value was obtained. Such an operation resulted in a large expenditure of both time and energy. Furthermore, it has been found that in the subsequent coating treatments the resin solids were no longer uniformly encapsulating the yarns, but instead were in effect coating the prior treatment. Microscopic examination of such fabrics has shown the creation of crystalized areas, i.e., the solid deposits of the resins cover more than one warp and filling yarn intersection. Thus, resin deposits were often found to be present on both sides of the fabric. The presence of such resin deposits on the front side of the fabric often marred the paper web, especially where thinner papers were being produced.